1. Field of the Invention
The present invention relates to a digital video system, and more particularly to a video receiving system that selects a desired channel from a plurality of channels and reproduces a digital video signal from a digital modulation signal of the selected channel.
2. Description of the Related Art
In an interactive television system (ITV) providing interactive services such as video-on-demand (which will be referred to as VOD hereinbelow), there has been recently developed a system in which a compressed video signal that is modulated according to a quadrature amplitude modulation scheme or the like is transmitted to a set-top box (STB) or a digital video selective-reproduction apparatus where the original video signal is demodulated and reproduced. In such a digital transmission system, a digital transversal equalizer employing a transversal filter is used in order to equalize amplitude delay distortion occurring in a transmission path, an up-converter, a tuner and others.
In such a set-top box, a tuner selects a desired carrier from a frequency-multiplexed input signal, and a digital modulation signal of the selected channel is demodulated by a QAM (or QPSK) demodulator. After being subjected to waveform equalization by the digital equalizer and to error correction, the demodulated signal is reproduced into a video signal by a digital signal processor which is output to a monitor. In general, channel selection of the tuner is performed by a user operating a remote control transmitter or an external switch so that the tuner receives a channel selection signal.
The digital modulation signals are usually frequency-multiplexed in several tens of frequency channels in steps of 6 MHz or 12 MHz, and the user performs a channel changing operation to select any desired channel among these channels. When changing the channel, however, a receiving frequency of the tuner is changed, resulting in a temporary input interrupt of the QAM demodulator. Control of the digital equalizer and a carrier recovery circuit of the QAM demodulator thus enters a divergent state, thereby requiring a relatively long time from input of a new channel signal to control convergence. Therefore the conventional receiver cannot achieve rapid channel changing.